Disk defragmentation

Files on hard drives become fragmented over time with disk usage due to the way that files are deleted and written to the hard drive. Information on a hard drive is stored in small units called clusters that are typically 512 bytes. Individual files bigger than 512 bytes are assigned multiple clusters by Windows. The clusters that make a file are not required to be next to each other (Windows keeps track of the location of all clusters that make up a file). Files become fragmented because clusters are reused. For example, when a two cluster file is deleted and then a four cluster file is written the four cluster file may be split. This split can occur because the two clusters of the deleted file are reused and if there are not two free clusters next to the recently freed two clusters, the four cluster file will have to be split. Multiply that scenario by the thousands of operation your computer does every day and you can imagine how you hard drive can become fragmented. File fragmentation slows down Windows because Windows has to race all over the place to access the various clusters that make up files. Reading from the hard drive is much more efficient if all the clusters that make up a file are right next to each other.

Using Disk defragmenter
It is a good idea to run the Windows Disk Defragmenter once a month. Disk defragmenter can be accessed by "right clicking" a hard drive icon in "My Computer" (Windows XP) or "Computer" (Windows Vista) and choosing Properties->Tools->Defragment Now. In Windows XP you have to run disk defragmenter yourself. In Windows Vista Disk Defragmenter is scheduled to run automatically on all of you hard drives once per week by default (it may be desirable to change this to once per month as once per week is more frequent than necessary).

Videos
Running and automating Disk Defragmenter in Windows XP.
Changing the schedule of Disk Defragmenter in Windows Vista .